A typical ratchet-type tensioner applies tension to a timing chain by exerting force on the chain by means of a plunger which is movable along an advancing and setback direction, in a plunger accommodating hole in the tensioner housing. The plunger is biased in the advancing direction by a spring and external hydraulic pressure provided by oil in a chamber formed by the tensioner housing and the plunger.
A typical prior art ratchet-type tensioner is disclosed in Japanese Utility Model No. 2559664, and shown in FIG. 16. In the tensioner 500, a plunger 514 is slidable in a plunger-accommodating hole in a housing 512, and protrudes from the housing, being biased in the advancing direction by a spring 518 and by oil pressure within a chamber 516 formed by the plunger and the housing.
A piston 526 slides in the housing 512 in a direction orthogonal to the direction in which the plunger 514 slides. An oil sub-chamber 520 is formed by the piston 526 and the housing 512, and an oil passage 544 supplies oil under pressure to the oil sub-chamber 520, urging the piston away from the plunger 514. A spring 534 biases the piston 526 toward the plunger, opposing the force exerted by the oil in the sub-chamber 520. Spring 534 is located within an air chamber 528 on the side of piston 526 opposite from the sub-chamber 520. An air hole 532 in communication with the air chamber 528 is closable by a rod 524, to which the piston 526 is attached, when the piston 526 is moved away from the plunger against the biasing force exerted by spring 534 by hydraulic pressure in sub-chamber 520 and the piston 526 moves against a biasing force of the second spring 534.
A rack of teeth 538 is provided on the plunger 514, and a plurality of ratchet teeth 536, capable of engaging the rack teeth 538, is provided at the end of rod 524 opposite form the end that is arranged to close off air hole 532. Surfaces of teeth 536 and 538 for blocking retraction of the plunger are formed at a right angle to the direction in which the plunger 514 moves.
The prior art tensioner requires precise assembly of the cap 530, which is sealed by press-fitting after the rod 524 has been installed within the air chamber 528. Thus, the cap 530 cannot be readily removed, preventing ready disengagement of the rack teeth 538 and ratchet teeth 536 for maintenance of the tensioner 500 and timing chain (not shown).
Further, because the prior art tensioner 500 comprises a plunger 514 and piston 526, engagement of the rack teeth 539 and ratchet teeth 536 causes a twist in the rod 524, which, as the plunger 514 vibrates while the engine is driving, can cause chipping of the teeth. In addition, the twist in the rod 524 can lead to faulty operation of the ratchet mechanism, and to erroneous assembly during manufacture and maintenance of the tensioner.
Because the tooth surfaces in the setback direction are perpendicular to the advance/setback direction of the plunger 514, setback of the tensioner is prevented even when it may be desirable, i.e., when tension of the chain becomes excessive due to temperature change and other causes. Thus, the prior art tensioner 500 experiences excessive tension, wear, noise and eventual seizing.
To alleviate the problems of excessive tension, wear, noise, etc., a predetermined backlash is provided in the ratchet mechanism, corresponding to a presumed maximum distance of the movement of the plunger 514 in the setback direction caused by the excessive tension. However, the larger the allowed backlash distance, the larger the “flapping noise” generated on starting the engine.
Prior steps to remedy these problems have been the addition of an orifice mechanism and an oil-reserve mechanism or replacement of the spring 518 with a higher-load spring. However, this increases the number of parts, the cost of production, and the size of the tensioner itself.